The present invention relates generally to the field of steam generators used in nuclear power plants. More specifically, the present invention relates to improved nozzle dams for hot and cold legs of a steam generator, as well as methods for installing and removing such improved nozzle dams. The present invention also includes methods and apparatus for the pressurization and control of steam generator nozzle dam seals.
Nuclear power plants are routinely shut down for refueling, maintenance, inspection, and testing. FIG. 1 shows a simplified diagram of a typical nuclear power plant 10 which includes a steam generator 12, a reactor pressure vessel 14 holding a reactor core 16 in a core support barrel 18, and a refueling pool 20. When refueling a nuclear power plant or servicing the reactor core 16, the reactor pressure vessel 14 and refueling pool 20 are flooded with water. However, when the reactor pressure vessel 14 and refueling pool 20 are flooded, water will typically enter the steam generator 12 preventing maintenance, inspection and testing of the steam generator 12 during refueling or servicing of the reactor core 16. In order to simultaneously service both the reactor core 16 and the steam generator 12, some form of temporary seal must be installed in the piping 22 connecting the reactor pressure vessel 14 with the generator 12 in order to isolate the reactor core 16 and refueling pool 20 from the steam generator 12, thus permitting simultaneous testing and inspection of the generator components. This seal is achieved by installing what is known in the industry as a “nozzle dam” in the nozzles of the steam generator primary head. A cutaway view of the nozzle 24 of the steam generator 12 is shown in FIG. 1A. The nozzle dam 26 is designed to be carried through a small manway 28 in the generator head and assembled by hand. As the nozzle dam installer is subject to radiation exposure inside the steam generator 12, the nozzle dam 26 must be installed as quickly as possible in order to minimize the radiation exposure. The nozzle dam 26 also must effect a reliable water-tight seal able to withstand high water pressures without compromising the structural integrity of the nozzle wall or steam generator wall.
FIG. 2 shows a cutaway view of a typical prior art nozzle dam 26. Such nozzle dams 26 used to seal the nozzles 24 of nuclear power plant steam generators typically use aluminum structures supporting a rubber diaphragm 30 with pneumatic seals (e.g., a dry seal 32, a wet seal 36, and an annulus 34 between the wet seal 36 and dry seal 32), as shown in FIG. 2. Two variations of nozzle dam attachment are currently in use. FIGS. 3 and 3A show cutaway views which depict the nozzle dam 26 attached to the nozzle 24 utilizing radial pins 38 interfacing with holes 40 on the interior of the steam generator nozzle 24 or interfacing with welded hold-down rings. As shown in FIG. 4, another common attachment method uses a flange 42 at the top of the nozzle dam 26 bolted to a ring 44 that has been welded to the steam generator bowl at the junction of the nozzle 24 and the body of the steam generator 12. The inside diameter of the welded ring 44 may also serve as a sealing surface for the pneumatic seals.
Other examples of prior art nozzle dams are described in U.S. Pat. No. 4,667,701 and U.S. Pat. No. 4,957,215.
Such prior art nozzle dam designs were designed as retrofits for pre-existing steam generators and were thus constrained by the pre-existing design of the steam generator nozzles. Accordingly, these prior art nozzle dams were limited in terms of placement position in the nozzle, attachment points and supports, unknown sealing surfaces of the nozzles, and limited manway openings. Further, such prior art nozzle dam installation technicians are subject to radiation exposure level limitations. These constraints resulted in nozzle dams that were large in size, heavy in weight, difficult and time consuming to install and remove, had unknown sealing surfaces, expensive to manufacture, comprised of multiple moving components such as structural bolts, pins or other locking mechanisms each of which had the potential for failure, and not readily adapted for remote installation or removal.
With the advent of new nuclear power plant designs, such as Westinghouse's new AP1000 nuclear power plant design and the supply of new replacement stream generators, an opportunity exists for overcoming most, if not all, the limitations of prior art nozzle dam designs by working with the steam generator manufacturer to ensure standardized steam generator nozzles with uniform sealing surfaces.
It would therefore be advantageous to provide a nozzle dam design for steam generators of newly designed nuclear power plants and for replacement steam generators, which when compared to the prior art nozzle dams are lighter in weight, smaller in size, simpler and quicker to install, have a known sealing surface, are economical to manufacture, are designed without multiple moving components such as bolts, pins, or other locking mechanisms having the potential for failure, minimize radiation exposure, and are adaptive to remote installation and removal.
The methods and apparatus of the present invention provide the foregoing and other advantages.